Evaluating the Performance of Engineered Barriers

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1 Evaluating the Performance of Engineered Barriers Outcomes of the PEBS project Annika Schäfers (BGR), Irina Gaus (Nagra), Klaus-Jürgen Röhlig (TU Clausthal) 1 1

2 Introducing PEBS Long-term Performance of Engineered Barrier Systems Project duration:

3 Introducing PEBS Objective Improve understanding of the early resaturation phase of the bentonite barrier Reducing associated uncertainties in longterm safety assessment 3 3

4 Introducing PEBS Procedure Identification of uncertainties during early evolution of EBS for different disposal concepts Laboratory and in-situ experiments on THMC processes in bentonite Numerical modelling of short-term effects and extrapolation to long-term evolution Analysis of impact on longterm safety and guidance for repository design Switzerland France Sweden Spain Germany [Source: PEBS D3.1-1] [Source: PEBS D3.1-1] 4 [Source: PEBS D1-1] 4

5 Introducing PEBS Identified uncertainties cases Cases: areas of process-based understanding that need to be evaluated to improve the integration in long term safety assessment Uncertainties associated with water uptake in bentonite for early evolution (T<100 C) Uncertainties associated with bentonite performance for early evolution (T>100 C) Hydro-Mechanical evolution of bentonite barrier Geochemical evolution of bentonite barrier 5 5

6 Outcomes of PEBS Selected results of analysed cases Uncertainties associated with water uptake in bentonite for early evolution (T<100 C) Results of enhanced numerical models showed good agreement to observed water uptake in large-scale heater experiments Uncertainty regarding late stage resaturation predicted with models Observations show that safety function of the buffer is fulfilled by sufficient swelling pressure even prior to full liquid saturation Existing model uncertainty not significant from long-term safety perspective 6 FEBEX mock-up Modelling of relative humidity in FEBEX mockup 6

7 Outcomes of PEBS Selected results of analysed cases Uncertainties associated with bentonite performance for early evolution (T>100 C) New in-situ experiment (HE-E): good agreement between models and measurements for thermo-hydraulic processes in EBS and host rock Resaturation is slow several years of monitoring required to adequately test models for resaturation Further studies on effects of heating bentonite above 100 C in partially saturated state suggest no significant changes in performance in the considered high temperature range (up to ~ 150 C) uncertainties remain regarding swelling and hydraulic properties of bentonite for peak temperatures above approx. 125 C [Gaus et al., 2014] 7 HE-E heater test at MT URL 7

8 Outcomes of PEBS Selected results of analysed cases Hydro-Mechanical evolution of a bentonite barrier EB experiment: effective sealing through swelling of bentonite blocks and pellets with large initial density differences Even though homogenization in the EB was reasonably efficient, some heterogeneities remain over the cross-section of the test Numerical modeling proved capable of predicting heterogeneities in the barrier after hydration, but involved coupled processes are complex Uncertainties remain regarding evolution of heterogeneities in the EBS 8 Concept of the EB experiment at MT URL [Source: PEBS D3.1-1] THM modelling of the porosity distribution in the EB experiment [Source: PEBS D3.1-1] 8

9 Outcomes of PEBS Selected results of analysed cases Geochemical evolution of bentonite barrier 9 Limited bentonite alteration at the interfaces (both steel-bentonite and cement-bentonite) over years (few cm reaction zone over very long term), porosity evolution requires further work below about 130 C, limited alteration of smectite is expected within the main part of the barrier based on alteration models and natural analogues (for short thermal phase) Geochemical modelling provides valuable insights but uncertainties exist regarding extrapolation to the long term further model testing and supporting information needed abstraction for PA purposes should be performed cautiously 9

10 Outcomes of PEBS Conclusions Constraining uncertainties related to resaturation period associated uncertainties in long-term performance of bentonite barriers reduced in some important areas. Improvements in process understanding and model development Arguments have been further developed concerning: Development of swelling pressure during slow wetting target swelling pressure reached before full saturation General minor impact of higher temperatures for short periods Performance of heated bentonite Swelling of bentonite blocks and pellets Constraining the impact of geochemical interactions at the EBS interface 10 10

11 Outcomes of PEBS Recommendations Existing and new in-situ experiments will play an important role in further confirming bentonite performance over periods of years For concepts anticipating temperatures above 100 C further data collection will be needed for certain aspects, e.g. swelling pressure, strength Further work on the geochemical processes at material interfaces is required potential evolution of density heterogeneities requires further research, especially for concepts where a heterogeneous inflow is expected 11 11

12 Progress in scientific domain Guidance by HLEC The High Level Expert Committee (HLEC) with members from regulators (Belgium, Sweden), research (Belgium) and academia (Germany) Focus on analysis of impact on long term safety (WP1 & WP4) but also interfaces with experimentation (WP2) and modelling (WP3) Outcome: recommendations concerning WP4 (HLEC Report) 12 12

13 Progress in scientific domain Guidance by HLEC 13 Conclusions and recommendations: Good science Linkage to safety-related questions not always obvious, though Address impact of uncertainties on safety functions more explicitly! Consider system as a whole (e.g. impact on canister corrosion), discuss how safety functions and barriers works together to ensure safety and robustness PEBS cases: choices driven by Most important uncertainties in terms of phenomenology / scientific understanding Common interest Good, but also discuss significance for the various repository concepts covered by PEBS! Address safety relevance more comprehensively in WP4! HLEC recommendations were addressed in WP4 and final reporting 13

14 Contribution to the IGD-TP vision and strategic research agenda Irina Gaus (Nagra) 14 14

15 Contribution to the IGD-TPs vision. PEBS was born before IGD-TP! But Strategic Research Agenda: Buffer and Backfill materials: The choice of buffer and backfilling materials is partly dependent on the chosen disposal concept. Buffer and backfilling have important safety functions in some disposal concepts and shall therefore fulfil the requirements and specifications set for them. As these depend on the disposal concept under consideration and the geological environment of the site their importance varies between the different waste management programmes

16 IGD-TPs Strategic research agenda Recognized in the projec t WP1 Describes the treatment of the early barrier evolution and in the safety assessments of a number of countries 16 16

17 IGD-TPs Strategic research agenda Common uncertainties over the programmes 17 Mass loss due to piping and erosion in the very early evolution Swelling and homogenisation of components with different density and sealing after losses of mass The importance of friction within the bentonite and between bentonite and other materials also in the unsaturated state Effects of temperature on the mechanical properties Hydraulic processes Mechanical behavior in contact with the metal elements Mechanical effect of gases inside Argillites and swelling clay Thermo-mechanical effects Technologies implemented in the repository: swelling clay seals and engineered barriers The effect of temperatures exceeding 100 C on the hydraulic properties of the buffer The evolution of swelling pressure with time and the interaction with the convergence of the host rock Chemical Interaction between bentonite and Fe and the low ph liner Discrepancy in the water saturation process of the buffer between predicted hydration rates and experimental values 17

18 IGD-TPs Strategic research agenda Buffer and Backfill materials: The processes occurring during repository evolution are similar for many materials and repository environments (e.g. compaction, diagenesis, changes in hydraulic characteristics), which makes comparisons meaningful, even though the impacts on disposal system performance may be significantly different. High importance and urgency Topic 10. Description of seals and plugs systems and modelling of their long-term behaviour with assessment of the consequences on long-term safety

19 IGD-TPs Strategic research agenda 4 CASES: advanced through experiments (WP2) and modelling (WP3) WP 1 Current treatment in PA Analyis of issues per repository concept Consolidation in cases Case 2 : Case 1: WP 4 Improvements Integration of new findings in national programmes, impact for specific concepts Feedback to design Remaining uncertainties Switzerland Sweden France 19 CASES: areas of process-based understanding that need to be evaluated to improve the integration in long term safety assessment 19

20 IGD-TPs Strategic research agenda Feedback to performance assessment: Nagra case Refinement of the early evolution placed in the correct spatial and temporal context to assess the consequences confirmation of assumptions of latest safety case (2002), input for the next (2022) Extrapolation modelling based on HE-E results (Kuhlmann et al., 2014) 1:2 Scale Heater Test at Mont Terri URL (Nagra, 2014)

21 Contribution to the IGD-TPs vision. Feedback to performance assessment: SKB SR-Site assessment (SKB 2011): if the buffer and backfill is installed as envisaged buffer density and swelling pressure will homogenise to a situation where the relevant safety functions will be upheld. Obervations EB experiment (Aitemin, Ciemat, 2014) PEBS confirmed that: - even if installed as a mixture of high density blocks and low density pellets a highly saturated bentonite barrier was able to seal all the initial voids. 21 Modelling of the EB experiment. (CIMNE, 2014) - FEBEX mock up shows that the buffer achieves the required performance over time as saturation increases, and will perform long before full saturation is reached. 21

22 Contribution to the IGD-TPs vision. Remaining uncertainty at the end of PEBS: For concepts where a heterogeneous inflow is expected, evaluation of the potential evolution of density heterogeneities during saturation might need further attention locked in heterogeneities. IGD-TP at work Working Group in this Exchange Forum HOMOBENTO next HORIZON2020 project? 22 September 2015: bentonite swelling into liner-heater gap after 18 years results from the FEBEX-DP project at the Grimsel Test Site (Kober et al., 2015) 22

23 Thank you! Thanks to the PEBS partners for the input to this presentation. The research leading to these results has received funding from the European Atomic Energy Community's Seventh Framework Programme (FP7/ ) under grant agreement n